comptime VM arc: abi(.compiler) ABI, out as sx fn, VM-native diagnostics, BuildConfig threaded

Lands the full VM/compiler-API arc on branch reify (701/0 both gates):
- abi(.compiler) ABI replaces abi(.zig) extern compiler + the fake
  #library "compiler"; bodiless decl = compiler-API surface, bodied =
  user compiler-domain fn (lowered for VM eval, emit-skipped).
- out is a plain sx fn (libc write) — the out builtin deleted; the VM
  handles it via host-FFI. trace_resolve + interp_print_frames ported.
- 4B VM-native diagnostics: 1179/1180 render proper comptime type
  construction failed: under strict.
- S5a: build_options/set_post_link_callback on abi(.compiler) with
  BuildConfig threaded into the VM (green intermediate).
- 0522 fixed (describe(args: []Type)); regression 0638.

Strict deletion-gate down to 4 compiler_call bails (1609/1614/1615/1616)
+ 1654 (legitimate unresolvable-symbol diagnostic).

src/ir/inst.zig

@@ -431,7 +431,6 @@ pub const BuiltinCall = struct {
 };
 
 pub const BuiltinId = enum(u16) {
-    out,
     sqrt,
     sin,
     cos,
@@ -577,13 +576,21 @@ pub const Function = struct {
     /// `__sx_ctx` value to the args of a call. Extern decls and
     /// `abi(.c)` functions have it false.
     has_implicit_ctx: bool = false,
-    /// True for a `fn abi(.zig) extern compiler` welded to the comptime
-    /// `compiler` library. Such a function has no real symbol — the comptime
-    /// interpreter dispatches it to its registered Zig handler
-    /// (`compiler_lib.findFn`) instead of dlsym. Comptime-only; a runtime call
-    /// has no backing symbol. See design/comptime-compiler-api.md.
+    /// True for a bodiless `abi(.compiler)` compiler-API function (`intern`,
+    /// `find_type`, `build_options`, …). Such a function has no real symbol — the
+    /// comptime evaluator dispatches it to its registered Zig/VM handler
+    /// (`compiler_lib.findFn` / `Vm.callCompilerFn`) instead of dlsym. Comptime-only;
+    /// a runtime call has no backing symbol (the `emitCall` gate rejects it).
     compiler_welded: bool = false,
 
+    /// True for a BODIED `abi(.compiler)` function — a user compiler-domain function
+    /// (e.g. a post-link callback). Unlike `compiler_welded`, it HAS an sx body the
+    /// comptime evaluator runs; but it NEVER runs in the shipped binary, so the
+    /// backend does not lower it (emit_llvm Pass 2 skips it, like `is_extern`). Its
+    /// only references are in comptime code (the `#run` that registers it) → DCE
+    /// drops the leftover declaration. See current/PLAN-COMPILER-VM.md (S3).
+    is_compiler_domain: bool = false,
+
     pub const Param = struct {
         name: StringId,
         ty: TypeId,